This invention concerns improvements in or relating to infra-red optical systems and relates more particularly to afocal optical systems for use at infra-red wavelengths.
Afocal optical systems, i.e. systems having an infinite focal length and therefore no apparent focussing action can be used where it is required that a beam of infra-red radiation from a distant source and incident on the optical system be reduced in diameter. Such reduction of beam diameter enables a comparatively small, high speed reflective scanning system to be employed in order to provide the requisite scanning of the field of view across an imaging lens plus line detector array.
It has previously been proposed to use for such infra-red optical systems a non-Galilean afocal telescope having an objective system which produces an intermediate real image that is viewed by an "eye-piece" system comprising two meniscus lens elements disposed with their convex surfaces facing each other (see U.S. Pat. No. 3,947,084). It will be understood that the term "eye-piece" is used in this context although the ultimate image is not viewed by the human eye but is, for example, scanned across an infra-red detector array. It has been found that an afocal infra-red telescope employing such an "eye-piece" system can suffer from problems of astigmatism, of improper match between the "exit pupil" of the objective system and the "entrance pupil" of the "eye-piece" system, and of the telescope not being truly afocal at all scan angles. There is a requirement for overcoming or reducing such problems, and for doing so without unduly increasing the aperture requirements in, and in front of, the objective system.